This invention relates to a method for setting density values of highlight and shadow points of original pictures in a picture reproducing machine such as a color scanner and a color facsimile system.
When color separation printing plates or films are prepared from an original picture by using a picture reproducing machine such as a color scanner, since the density range of the original picture is usually different from the reproducible density range of the picture reproducing machine, proper highlight and shadow points are selected in the original picture, and then their densities are set in the picture reproducing machine, thereby adjusting the density range of the original picture to the reproducible density range of the picture reproducing machine.
Such a setup of the density values of the highlight and the shadow points have been carried out in several ways. For example, the density values of the highlight and the shadow points of the original picture are detected by a densitometer. Then, a sensitivity of a photomultiplier tube is controlled in order to adjust the density value of the highlight point to a standard highlight density predetermined, and the density value of the shadow point is set up by adjusting a variable resistor of a density range setup circuit for setting up the density range of the original picture. Alternatively, as shown in FIG. 1, the density values of the highlight and the shadow points are set up in a density range setup circuit having a plurality of resistors selected by using rotary switches S.sub.1 and S.sub.2 connected thereto.
However, in the former method, the reproducibility of the highlight and the shadow points is not very good when the density values are detected, and a plurality of original pictures having different highlight and shadow point density values cannot be processed in a single operation. In the latter method, the improvement of accuracy of the setup or the extension of the setup range requires an excessive number of switches, and, when the switches are to be switched over at a high speed, viz., a plurality of original pictures having different highlight and shadow point density values are to be scanned, it is difficult to apply this method to the picture reproducing machine.